These file(s) are part of the Werner Icking Music Collection. Note: SBB abschrift SBB Mus.ms. 6711 (copy from ca.1790) gives no textual support for the B♭ (or rather G♮) in measure 5. Perhaps an earlier/better source does. What is this version's sources? You mentioned, of course (all WIMA typesets do, obviously) but I forgot.